An amplitude modulated signal received over a transmission medium can be distorted due to transmission characteristics of the transmission medium, resulting in attenuation of the signal at different frequencies. A receiver typically includes an equalizer for compensating for the distortion due to the frequency response of the transmission medium. Ideally, the frequency response of the transmission medium should be flat; that is, the magnitude of the sidebands should be equal. However, the frequency response can be downward sloping or upward sloping dependent on frequency and phase distortion in the received signal due to the unequal magnitude of the sideband carriers. The frequency response of a filter in the equalizer is generally selected to approximate the inverse frequency response of the transmission medium in order to equalize or “flatten” the frequency response.
The characteristics of the transmission medium or the apparatus used to correct the received signal may vary with time or temperature. Thus, in that case, the frequency response of the equalizer must adapt to varying distortion in the received signal. The frequency response must be continuously measured and the inverse frequency response of the equalizer modified in response to changes in the measured frequency response. One known method for adaptively adjusting the inverse frequency response is to directly measure the frequency response of the transmission medium. However, prior art equalizers with adaptive frequency response based on direct measurement of the frequency response of the transmission medium require characterization of the medium for example, using test tones to “flatten” the frequency response. This may be technically inconvenient, if not impossible.